1. Field of the Invention
This invention relates to an apparatus and method for filtering liquids containing suspended solids. Specifically, this invention relates to an improved vertical filtration unit.
2. Description of Related Art
The removal of suspended solids from liquids, especially water, has been a longstanding requirement for many uses of such liquids. For example, water, which is free of suspended solids, is greatly preferred for uses, such as drinking, industrial processing, and swimming pools. Vertical filters, which utilize filter media, such as sand, have been used to separate suspended solids from water.
Vertical filters operate by introducing the liquid containing suspended solids onto the top portion of a bed of filter media, usually sand. As the liquid passes through the filter media under the force of gravity, the suspended solids adhere to the filter media, allowing liquid free of suspended solids to accumulate and exit from the bottom of the vertical filtration unit.
The above-described method, although effective at removing suspended solids from liquids, has several shortcomings. The filter media eventually becomes saturated with suspended solids and no longer effectively removes suspended solids from the incoming liquid. When this condition occurs, the vertical filtration unit must cease operation so that the captured suspended solids can be removed from the filter media. This removal is accomplished through an operation referred to as xe2x80x9cbackwashing,xe2x80x9d in which clean liquid is introduced, under pressure, to the bottom of the filter media bed and passes through the filter media in the reverse direction from the filtration process. The reverse flow of the liquid removes the filtered solids from the filter media and exits through the top of the vertical filter. This operation is typically continued until the exiting backwash water is free of suspended solids, indicating that the filter media is clean.
Backwashing can be very time consuming and generates large volumes of wastewater. Additionally, filtered solids are never completely removed from the filter media during backwashing, which eventually results in the filter media needing to be replaced. Filtration devices utilizing the above method are disclosed in U.S. Pat. No. 650,611.
The problem of backwashing filter media has been addressed, to some extent, through the development of continuous or moving bed filtration systems. While these continuous filtration systems operate to remove suspended solids as outlined above, they have the added benefit of continuously removing filtered suspended solids from the filter media. In these methods, the dirtied filter media at the bottom of the vertical filter unit and a small amount of filtered water are airlifted to a washbox. This lifting is carried out by an airlift tube equipped with a compressed air supply. The mixture of filtered water, filtration sand, and suspended solids is directed against a baffle or plate to scrub the suspended solid particles from the sand.
The washed sand is returned to the top of the filter media bed. The suspended solid particles, which are washed from the filter media, are flushed from the filter by a combination of filter effluent liquid and filtered liquid that carry the suspended solids across a reject weir and out of the vertical filtration unit. The filtration cycle automatically repeats itself with the influx of a new supply of liquid to be filtered.
Examples of the above-described continuous filtration systems are disclosed and explained in U.S. Pat. Nos. 4,060,484; 4,891,142; and 5,582,722, all of which are herein incorporated by reference.
Conventionally, filtered effluent is removed by way of exit pipes that require cutting holes into a filter vessel or concrete basin. This can create difficulties and inconveniences when existing basins, with no pre-existing holes, are retrofitted for filtration use. It is desirable in many cases to not bore holes in the side of such basins.
The above-described continuous systems for the filtration of liquids and the removal of suspended solid particles are typically not as effective or efficient as industry and the public""s need and desire. These systems typically require level sensors and valve actuators, which are expensive and fail when electrical power supply is lost. Many valves are often required which can cause confusion for those operating the filtration unit. The airlifting operation often xe2x80x9cplugsxe2x80x9d with filter media, causing the filtration unit to be shut down for cleaning. There remains an unmet need to provide a reliable, maintenance-free, continuous, self-cleaning, vertical filtration unit for the removal of suspended solids from liquids that may be retrofitted to existing basins with no requirement for boring holes into the basin.
The present invention is directed to a liquid filtering apparatus that includes a washbox, an airlifting tube, a central pipe, an effluent riser pipe, an effluent chamber, at least one screen cartridge in communication with a lower portion of the effluent chamber, a reject outlet pipe communicating the washbox with a reject box, and an effluent collector communicating the effluent chamber with a location outside of the liquid filtering apparatus.
The airlifting tube extends from the washbox and includes an expulsion end within the washbox and an intake at a lower end, opposite from the washbox. The central pipe extends along an outside surface of the airlifting tube from a position just below the washbox to a position just above the intake end. The effluent riser pipe extends along an outside surface of the airlifting tube from a position just below the washbox to a position above the intake end. The effluent chamber is defined by the outer surface of the central pipe, a bottom plate of the effluent riser pipe, an upper plate of the effluent riser pipe, and an inner surface of the effluent riser pipe. The at least one screen cartridge is oriented in a generally angular, downward direction.
Generally, the liquid filtering apparatus is placed in a vessel having a bed of filtration material and an intake for introducing unfiltered liquid into an upper region of the vessel. The vessel may be a pre-existing vessel to which the liquid filtering apparatus is retrofitted.
The present invention is also directed to a method of filtering liquid. The method includes the steps of: (a) placing a liquid filtering apparatus in a vessel having a bed of filtration material and introducing unfiltered liquid into an upper region of the vessel; (b) filtering the unfiltered liquid through the bed in a downward direction; (c) collecting filtered liquid in at least one screen cartridge and conveying the filtered liquid upward into an effluent chamber; (d) removing the filtered liquid from the effluent chamber by way of an effluent collector; (e) withdrawing a mixture of filtered liquid and dirtied filtration material from a lower portion of the bed of filtration material by way of an airlifting tube; (f) washing the dirtied filtration material by deflecting the mixture off of a surface and separating the dirt from the filtration material; (g) collecting the dirt and liquid separated in step (f) and removing it by way of a reject line; and (h) depositing the washed filtration material to the top of the bed of filtration material.
The present invention is further directed to a liquid filter assembly wherein two or more of the liquid filtering apparatuses described above are included.